What I really love about Microsoft Paint on Windows is to have a re-size canvas option (see attached picture). Actually it is must have feature in MS-Paint because there is no crop image option, but it would be nice if this re-size canvas option is available in Pinta.

I don't know how hard is this to implement. Maybe it can be done in similar way as:
1. you already know where mouse position is - you have this info in last icon from toolbar.
2. then just use "Rectangle Select" from position 0,0 and the mouse position (from step 1)
3. use "Crop to selection" from toolbar.